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I really don't think it's a thermostat issue in my 96 Legacy 2.2

Heater/defroster is only blowing cold air until I drop it down into a lower gear and get the RPM's up above 3500 or higher. I get normal hot air coming through the duct's then. I stopped and popped the hood while in park at an idle and looked in the coolant tank and low and behold I see air bubbles coming up at about the same rate as a goldfish aquarium.

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Sounds like headgaskets. Let it cool, Jack the front as high as you can or put it on a hill. Fill the radiator and overflow bottle. Start the car and burp the coolant by squeezing the upper radiator hose until the bubbles stop coming out and then put the radiator cap back on.

Now if you blow coolant out of the overflow bottle you most likely have a HG leaking. You can check it with a chemical tester as well. NAPA has them if you want to do it yourself.

+1 on head gasket trouble. I had the same trouble last Fall. Once my temp gauge shot up to hot, I pulled off the street, shut off the motor, opened the hood to look for bubbles in the over flow tank. Sure enough it was bubbling like an aerator in an aquarium. The bubbles is/are exhaust gas blowing by the bad head gasket to enter the cooling system. The water pump can't pump an air bubble, so the heater/defroster reacts by blowing cold air.

I'm going to burp the old girl and do a compression test and take a look at the spark plug's this week. At least there isn't evidence of water in the oil at the moment and I have not done any work on any of the cooling system component's, thermostat, etc. I did notice today that when the heater isn't working only one heater hose at the firewall is hot and the other one on the other side is cold. Airlock in the heater core possibly? Both hoses do get hot after I run it at high RPM's for a couple minute's though.

I'll go to Napa and get a tester after I burp it and get that big air bubble out of there. Perhaps that's the main cause of the bubbles coming up in the coolant tank? I know the previous owner said he had to refill the radiator about every four hundred miles or so because it was losing coolant. I'd found the source of the coolant loss he mentioned very quickly after I'd driven the car home from Ballard last summer. It was just a leaking plastic burp screw on the top passenger side of the radiator. I replaced it with a new one and haven't had to add any 50/50 ever since. The old girl has probably had a bunch of air in her all along? Ha Ha. I'll find out here shortly. Thanks for the help people. I appreciate it.

Probably just your heater control valve not opening completely. The extra revs give the water pump a boost and push hot water past the valve. Also will cause a perfectly good head gasket to pass an occaisonal air bubble from the hydraulic back pressure you're building up. All gaskets leak - it's just a question of whether or not it is an acceptable quantity.

Sorry, been a long time since 90-94. Should be easy enough to check flow by cutting the heater core out of the loop by disconnecting the hoses at the heater core nibs at firewall and tying them together to complete the coolant circuit. Clogged heater core would create the same symptoms.

I have a compressor. Would forcing air backward's through the heater core clear it (if it's partially plugged) or damage it? I don't know if the previous owner had dumped any stop leak additive's into it when he was experiencing his mindboggling coolant loss? That may be a possibilty.

I must fess up that I haven't checked the radiator since last summer. It took almost an entire gallon of 50/50 just to top off the radiator. I put the front of the wagon up in the air and let it run for about a half hour and watched air bubble up out of the radiator. I took it for a forty mile trip on I-5 and the heater works better now but still blows much cooler air at an idle.
ALSO:The plastic coolant tank is bubbling worse than it was before. My best hope is a bad seal on the water pump as the culprit but I have a feeling it's the HG's? I'll put some of that combustion leak detector in the overflow tank and see if it change's color. I can do it that way right? 96 2.2

I'd found the source of the coolant loss he mentioned very quickly after I'd driven the car home from Ballard last summer. It was just a leaking plastic burp screw on the top passenger side of the radiator.

Ironically, that was probably helping the situation. Because it leaked there, Exhaust gasses and bubbles in coolant escaped, and no pressure built up, so coolant still circulates.

When you close the system, the pressure builds, the bubbles are trapped, and the coolant stops circulating when too big an air pocket builds up.

I've loosened that screw a hair to limp home severely blown headgasket cars. It works, until you need to fill the system again.

Upper radiator hose blew up last night while driving back from town . Luckily I was only a quarter of a mile from my driveway when it went. It's obvious what contributed to that. Temp gauge never reached the half way point.

I put on a new radiator hose along with a gallon of Prestone antifreeze. I added the antifreeze via the upper radiator hose and I burped it good and drove around for a half hour without it getting hot but I'm still getting alot of persistent air bubbling up at the filler neck of the radiator when it's idling.
It's a no-brainer.

Autozone in my area want's $23.99 for the Felpro headgasket but their website doesn't specify if that's the price for one or both? And they state that new replacement head bolts are recommended. If I order the complete head gasket kit with new bolts I'll be packing a thin wallet for awhile. Is there an economical way of replacing the head gaskets and still getting a quality result?

Head gaskets come one to a pack. So that's $ 23.99 each.
You DO NOT need new head bolts. Subaru head bolts are not stretch type. Don't let the parts person talk you into buying them.
You will want new intake manifold gaskets, usually exhaust manifold gaskets can be re-used. Single port exhaust gaskets are hard to come by in parts stores. They're $8-10 each at a dealer.